Aftermath products, paticularly the Kraken, are a decent choice for a inexpensive first time gun. They usually run about $170ish depending on the retailer. Though they may actually shoot alittle harder then you'd want at around 370 fps.
Most of us want higher fps rather then lower due to longer range and the ability to use heavier ammo to resist the wind better but for backyard combat, unless you have a really big back yard, 370 fps wouldn't be advisable. For that fps I'd advise a minimum engagement distance of at least fifteen feet. Probably more like twenty.